It is known to selectively derive a movement of an objective from two different drive sources such as a motor and a manual adjusting ring. U.S. Pat. No. 4,200,377 discloses a focusing mechanism for a camera objective wherein the drives are decoupled via a slip clutch. Published German patent application 4,331,650 discloses a planetary transmission. The inactive drive is brought to standstill by the action of a friction brake.
A manual focusing by adjusting the object spacing (object to lens distance) with two different transmission ratios (coarse and fine drives) is normal for microscopes (that is, very different mechanical conditions); however, this manual focusing is exceptional for photo and film objectives and is only purposeful in a given area of application, namely, for a long focal length telephoto lens for sport (rapid) and nature photography (fine). Photo and film objectives usually have a single distance rotation ring. For rapid focusing, a small rotational angle is advantageous and for sensitive focusing a larger rotational angle is advantageous.
Various manufacturers of objectives offer therefore a range switchover especially for long focal length objectives wherein, with a switch or rotational ring, a distance range can be preselected and thereafter a sharp focusing is made within this range with a distance rotational ring over a larger rotational angle. However, this requires that the photographer must know ab initio in which range the work will be performed.
This is most of all critical at the range boundaries. If the photographer has preselected a range and attempts to sharply focus the object, but cannot attain the point of sharp focus because of the range boundary, the photographer must first change the range and then again search for the point of sharp focus with the distance ring. This is inconvenient and time consuming.